If you are using a .htpasswd file, edit the .htpasswd file to delete the line starting fred:

Warning: Do not use the Apache htpasswd program with .htpasswd files generated by TWiki! htpasswd wipes out email addresses that TWiki plants in the info fields of this file.

Remove the FredQuimby - fred line from the Main.TWikiUsers topic

Remove FredQuimby from all groups and from all the ALLOWWEB/ALLOWTOPIC... declarations, if any.Note: If you fail to do this you risk creating a security hole, as the next user to register with the wikiname FredQuimby will inherit the old FredQuimby's permissions.

Note: Consider leaving the user topic file in place so their past signatures and revision author entries don't end up looking like AnUncreatedTopic.
If you want to make it clear the user is no longer around, replace the topic content with a note to that effect.
The existence of the UserName topic should also prevent that user name from being re-used, sealing the potential security hole regarding inherited permissions..

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If you are using a .htpasswd file, edit the .htpasswd file to delete the line starting fred:

Warning: Do not use the Apache htpasswd program with .htpasswd files generated by TWiki! htpasswd wipes out email addresses that TWiki plants in the info fields of this file.

Remove the FredQuimby - fred line from the Main.TWikiUsers topic

Remove FredQuimby from all groups and from all the ALLOWWEB/ALLOWTOPIC... declarations, if any.Note: If you fail to do this you risk creating a security hole, as the next user to register with the wikiname FredQuimby will inherit the old FredQuimby's permissions.

Note: Consider leaving the user topic file in place so their past signatures and revision author entries don't end up looking like AnUncreatedTopic. If you want to make it clear the user is no longer around, replace the topic content with a note to that effect. The existence of the UserName topic should also prevent that user name from being re-used, sealing the potential security hole regarding inherited permissions..